The present invention relates to a method for monitoring the function and emergency braking capability of an electro-hydraulic braking system (EHB) comprised of a pedal-operated master cylinder and a pressure source with a hydraulic pump, with an accumulator, and with electrically controllable hydraulic valves, wherein the brake pressure in the wheel brakes is supplied from the pressure source in normal operation, and wherein a switch-over to a hydraulic connection between the master cylinder and the wheel brakes is executed in the event of an emergency braking mode.
There is a growing interest in electro-hydraulic brake systems (EHB) wherein the driver""s braking demand that is expressed by applying the brake pedal during normal operation is converted into an electrical actuating signal used to adjust and control the brake pressure in the individual wheel brakes by means of an electrically controllable hydraulic unit. This hydraulic device comprises one or more electrically driven hydraulic pumps, accumulators, and electrically controllable hydraulic valves that are used to proportion the wheel brake pressure for each individual wheel. Besides, the hydraulic unit comprises sensors for measurement and feedback of the hydraulic pressure that prevails at different locations in the system, among others, in the wheel brakes, at the outlet of the accumulator, etc.
Brake systems of this type provide an emergency braking capability even if the electronic unit and the pressure source fail. To reach this aim, the brake pedal acts upon a master cylinder which, in an emergency braking mode, instead of the pressure source, is connected directly to the wheel brakes by way of normally open hydraulic valves, i.e., open in their de-energized inactive position. In principle, there is no difference in the emergency braking mode compared to a conventional uncontrolled hydraulic brake system.
The result of the hydraulic uncoupling of the brake pedal from the wheel brakes during normal operation is that also the reactive effect of the braking action to the brake pedal is interrupted. While in conventional hydraulic brake systems the driver, when applying the brake pedal, monitors the operability of the brake system by observing the performance of pedal travel, pedal force, and vehicle deceleration, and thereby detects any leakage, an excessive clearance, or the inclusion of air in the hydraulic fluid or in the brakes by a change in the pedal travel or pedal feeling, this is no longer possible in an EHB in consequence of the uncoupling action. Failure of this monitoring function will lead to the circumstance that some defects cannot be recognized at all or not in time. This applies, among others, to the detection of air inclusions in the wheel brakes necessitating an increased pressure fluid requirement during a braking action which, in the normal mode, can be satisfied easily by the pressure source, but which cannot be complied with by the master cylinder in the emergency braking mode. This means, a xe2x80x98sleepingxe2x80x99 defect is caused which will not show until failure of the service brake and return to the fall-back level, the emergency mode.
In view of the above, an object of the present invention is to develop a monitoring method which permits detecting the loss in emergency braking capability, e.g. as a result of air inclusions in the wheel brakes, even if the brake system is intact.
It has been found that this object can be achieved by monitoring the function and emergency braking capability of an electro-hydraulic braking system (EHB) comprised of a pedal-operated master cylinder and a pressure source with a hydraulic pump, with an accumulator, and with electrically controllable hydraulic valves, wherein the brake pressure in the wheel brakes is supplied from the pressure source in normal operation, and wherein a switch-over to a hydraulic connection between the master cylinder and the wheel brakes is executed in the event of an emergency braking mode, wherein [the method [described in claim 1, the special characteristics of which includes that with predetermined events, e.g. during initialization of the brake system,] the emergency braking capability is checked by means of the following steps:
a) charging of the accumulator, without brake pressure buildup in the wheel brakes; measuring the accumulator pressure, and/or the accumulator volume or the filling level of the accumulator;
b) pressure introduction into the wheel brakes until a predetermined pressure threshold is reached, with the hydraulic pump deactivated or hydraulically uncoupled;
c) measuring the accumulator pressure;
d) determining and evaluating the accumulator performance in response to the pressure introduction into the wheel brakes for finding out the current volume requirement and/or the condition of the brake fluid, detecting air inclusions, etc., and for judging the current emergency braking capability of the brake system.
Thus, the present invention is based on the reflection that xe2x80x98sleepingxe2x80x99 defects of the type described above can be detected by testing the braking system in a simple and reliable manner using the method of the present invention during initialization of the brake system, that is, with each start of the vehicle.